The present invention relates to a gain stage of a differential amplifier used in a communication receiver at an intermediate frequency. Specifically, a gain stage is described which accommodates large differential input signals while maintaining high linearity.
A differential amplifier, as seen in FIG. 1, is effective at amplifying signals having a low differential, voltage swing. Differential voltages, within a limited voltage and frequency range, applied to the inputs of the amplifier will be linearly amplified to produce a larger differential voltage at the collector outputs 26, 27. The collectors of transistors 20, 21 are connected to a source of voltage VDD by resistors 23 and 24, respectively. The emitters are connected to a current source 25.
FIG. 2 is illustrative of the voltage gain property provided by the differential amplifier of FIG. 1. The differential output voltage appearing at the collectors of transistors 20 and 21, for a given differential input voltage, is plotted along the vertical axis and the differential input voltage is plotted along the horizontal axis. For differential input voltages between xe2x88x92Vt and Vt, the differential output voltage is substantially linear. Vt is the product of Boltzmann""s constant, k, and the device absolute temperature, T, divided by the elementary charge, q, and is approximately equal to 0.0259 volts at room temperature. As may be seen from FIG. 2, dynamic range of the amplifier is defined to be the largest input signal for which the output is not yet clipped, i.e. +/xe2x88x922Vt.
Large differential voltages applied to the differential amplifier cause distortion in the output signal. For differential input signals having absolute values increasingly greater than Vt, the voltage amplification of the differential amplifier becomes increasingly non-linear, as illustrated in FIG. 2. Differential input voltages having absolute values exceeding 2Vt produce non-linear, differential output voltages that are clipped at a maximum absolute value. The collector currents of the two transistors, 20 and 21, operate independently when the absolute value of the differential input voltage exceeds 2 Vt, since all of the amplifier""s current is flowing in only one transistor.
In some circuit applications, it is desirable to produce a substantially linear amplification of differential input signals outside the domain of xe2x88x92Vt to Vt. However, this is not possible with prior art differential amplifiers of the type illustrated in FIG. 1.
The differential amplifier of the present invention produces a substantially linear, differential output signal for a differential input signals exceeding xc2x12Vt without producing a clipped output signal. Increasing the dynamic range of the input signal is accomplished by connecting a regulation element with a pair of transistors forming a differential amplifier. Each transistor of the pair is connected to a first terminal of a power source through a separate impedance element. A current source connects the pair of transistors to a second terminal of the power source. The regulation element is connected between the first terminal of the power source and the current source and has an input connected to base terminal of the differential amplifier transistor.
A gain stage of this type reduces the gain of a differential amplifier without affecting the input and output impedances of the amplifier circuit, but ensures that a portion of the tail current flows in the load resistors even when the input differential signal reaches xc2x12Vt thereby significantly increasing the dynamic range of the source.